Certain motor vehicles, in particular heavy trucks and omnibuses, are usually equipped with a compressed-air brake system and sometimes also with an air suspension system. Thus, they have a supply system for the production, drying and storage of compressed air. When such a motor vehicle is equipped with an automated friction clutch, this is expediently also actuated by compresses air, i.e. engaged and disengaged by means of a pneumatic control cylinder, since the required energy source is already available in the motor vehicle.
DE 30 28 251 C2 describes a corresponding pneumatic actuating device for a friction clutch, which comprises an air inlet valve, an air outlet valve and a switching valve. The air inlet valve is in the form of a 2/2-way magnetic switching valve which, in its non-actuated rest condition is closed and which is opened to disengage the friction clutch. The air outlet valve is also in the form of a 2/2-way magnetic switching valve which, in its non-actuated rest condition, is closed and which is opened for the rapid engagement of the friction clutch, whereby a connection formed when the switching valve is in its non-actuated rest condition, vents the pressure chamber of the control cylinder, the air outlet valve and a throttle positioned downstream from the latter. The switching valve is in the form of a 3/2-way magnetic switching valve which, to set a particular torque of the friction clutch that acts when it is frictionally slipping while the air outlet valve is closed, is actuated cyclically whereby the pressure chamber of the control cylinder is vented via a throttle with a smaller cross-section connected downstream from the switching valve, so enabling relatively precise adjustment of the torque transmitted by the friction clutch.
Since magnetic switching valves are characterized by comparatively poor control dynamics, for the control of a friction clutch with which a defined disengagement position and hence a particular transmitted torque of a friction clutch is set in a regulating process by alternatively opening an air inlet valve or an air outlet valve, it is preferable to use pulsed valves, whose control dynamics are substantially better.
In an actuating device of this type the air inlet valve and the air outlet valve can be operated with pulse width modulation. In that method the effective opening level of each pulsed valve and therefore the control pressure in the pressure chamber of the control cylinder is adjusted by varying a time fraction (pulse width) within a constant cycle time. However, this control method has the disadvantage that, as experience shows, when the opening level is high undefined floating conditions of the magnet armature concerned can occur at the end of each time cycle, which impair the control dynamics and the ability of the valve to be regulated.
For that reason, in such an actuating device the air inlet and air outlet valves are preferably operated with pulse frequency modulation. In that method the effective opening level of each pulsed valve and hence the control pressure in the pressure chamber of the control cylinder is adjusted by varying the cycle time with a constant pulse width. In this case, at the end of each cycle time the magnet armature always reaches the end position that corresponds to the closed, rest condition, which results in increased control dynamics and improved regulation ability.
EP 0 459 273 A1 describes how a definite disengagement position of a pneumatically controlled friction clutch is set in a regulating process as a function of the difference between a specified nominal position and an actual position detected by sensor means, of the control piston of the control cylinder by alternative opening of the air inlet and the air outlet valves. The air inlet and air outlet valves are operated with pulse width modulation, the pulse width in each case being proportional to the difference between the nominal and actual positions of the control piston.
Finally, EP 0 512 690 B1 describes a pneumatic actuating device of a friction clutch, which comprises two air inlet valves arranged in parallel and two air outlet valves also arranged in parallel, each of them with a different opening cross-section. By actuating one or both air inlet or air outlet valves, it is thus possible to obtain different disengagement and engagement rates. Furthermore, by way of a hydraulic transmission device comprising an emitter cylinder, a pressure line and a receiver cylinder as well as a release lever, the control piston of the pneumatic control cylinder is in controlling connection with the contact pressure spring of the friction clutch. The stated objective of EP 0 512 690 B1 is to combine the pneumatic control cylinder, the emitter cylinder of the hydraulic transmission device and the air inlet and air outlet valves in a shared housing.
Particularly when starting off and also when stopping on an incline with the clutch slipping, a definite disengagement position and hence a definite transmitted torque of the friction clutch must be set as accurately and quickly as possible. It is true that this can be done by controlled actuation of air inlet and air outlet valves in the form of pulsed valves, since in a manner known perse, if pulse width modulation is used the pulse width, and if pulse frequency modulation is used the cycle time, are varied in proportion to the difference between the nominal and actual positions of the control piston or of a transmission element arranged between the control piston and the contact pressure spring of the friction clutch. It has been shown, however, that for this vary many regulation cycles are required and the air inlet and air outlet valves have to be actuated a correspondingly large number of times. Considered over the envisaged service life of the motor vehicle this can lead to premature wear of the air inlet and air outlet valves, which results in leakage-related problems of the actuating device and on the whole to impaired control characteristics of the friction clutch.